1. Field of the Invention
The invention relates to an exhaust gas control apparatus of an internal combustion engine.
2. Description of the Related Art Japanese Examined Patent Publication No. 1-27246 describes an internal combustion engine provided with an exhaust gas control apparatus. This internal combustion engine has two sets of cylinders, with three cylinders in each set. Each set of cylinders is connected to an independent exhaust pipe and a catalyst is arranged in one of these exhaust pipes. The other exhaust pipe, i.e., the exhaust pipe without the catalyst, is connected at the downstream end portion to the exhaust pipe in which the catalyst is arranged, i.e., the exhaust pipe with the catalyst. Further, the exhaust pipe without the catalyst is connected via a communicating pipe upstream of the catalyst to the exhaust pipe with the catalyst. As a result, exhaust gas discharged from the cylinders into the exhaust pipe without the catalyst can flow into the exhaust pipe with the catalyst both upstream of the catalyst via the communicating pipe and downstream of the catalyst without passing through the communicating pipe. That is, in the internal combustion engine described in Japanese Examined Patent Publication No. 1-27246, exhaust gas that was discharged from the cylinders into the exhaust pipe without the catalyst may flow through the catalyst and may also bypass the catalyst.
If the exhaust gas control catalyst which purifies components in the exhaust gas overheats, the purification efficiency of the exhaust gas control catalyst decreases, as described in Japanese Utility Model Application Publication No. JP-U-1-173423, for example. Therefore, there is a need to prevent the exhaust gas control catalyst from overheating in an exhaust gas control apparatus of an internal combustion engine provided with an exhaust gas control catalyst. Also, one known exhaust gas control catalyst purifies exhaust gas when the temperature of the exhaust gas control catalyst is within an appropriate temperature range. Thus, there is a need to control the temperature of the exhaust gas control catalyst to within the appropriate temperature range in an exhaust gas control apparatus of an internal combustion engine provided with this kind of exhaust gas control catalyst. Further, it may be necessary to raise the temperature of the exhaust gas control catalyst in order to achieve a specific objet. Moreover, it may be necessary to lower the temperature of the exhaust gas control catalyst in order to achieve a specific object.
There is also a need for an exhaust gas control apparatus of an internal combustion engine provided with two cylinders, which allows exhaust gas discharged from both cylinders to flow into an exhaust gas control catalyst.